Polypropylene films are widely used in the packaging industry because of their superior physical properties, such as high tensile modulus and stiffness, excellent optical clarity, and good moisture barrier characteristics. However, unmodified polypropylene has the disadvantage of having a high inherent coefficient of friction and film-to-film destructive blocking on storage. In the past, the coefficient of friction characteristics of polypropylene and other thermoplastic films have been beneficially modified by the inclusion in the polymer of fatty acid amides. See, for example, U.S. Pat. No. 3,176,021 for such a modified material. The effectiveness of the amides relies upon their ability to migrate to the surface of the films in order to reduce the coefficient of friction. While such amides do improve the coefficient of friction of the films, the value of the coefficient of friction is subject to wide variation depending upon the heat history which the film experiences during shipping, storage, and certain converting processes. In addition, the presence of such amides on the film surfaces can adversely affect the film's appearance as manifested by an increase in haze, a decrease in gloss and the presence of streaks. The presence of such amides on the surface can also adversely affect the wettability and adhesion of solvent and water-based inks, coatings, and adhesives.
In U.S. Pat. No. 4,533,509 to L. J. Gust et al, issued Aug. 6, 1985, entitled LOW COEFFICIENT OF FRICTION FILM STRUCTURE AND METHOD OF FORMING THE SAME, and U.S. Pat. No. 4,618,527 to R. H. Doyen, issued Oct. 21, 1986 entitled GOOD MACHINABILITY FILM STRUCTURE, there is described a multi-layer structure comprising a comparatively thick base layer of an optically clear thermoplastic resin and a comparatively thin surface layer containing a finely divided inorganic material in a proportion sufficient to thereby impart anti-block characteristics and decrease film-to-film coefficient of friction. The resulting film has non-blocking and improved slip characteristics which are stable regarding heat history without any adverse effects on appearance, wetting and adhesion as typically experienced with amide-modified films. However, when laminated to other films, e.g. glassine paper, such structures exhibit significantly higher coefficient of friction values and do not perform flawlessly on conventional form, fill and seal machines.